PURPOSE: To relax a posture in a reclining condition by connecting a base and a seat main body together by a relative movement guiding mechanism, providing the relative movement guiding mechanism with a fixing means for a guiding member, and providing a connecting means for the base and a seat part so as to keep a posture of the seat part constant in the case of a shift of the seat main body.

CONSTITUTION: A seat main body 3 and a base 6, which holds the seat main body 3 movably, are connected together by a relative movement guiding mechanism 7. The relative movement guiding mechanism 7 consists of a combination of a guide rail arranged in the base 6 and a guide member 9 arranged in the seat main body 3. A lock member 18 is constructed of plural engaging holes 15, 16,... arranged in the guide rail 8 and an engaging projection 17 arranged in the guide member 9. The engaging projection 17 is energized in the insertion direction by a spring 20. In addition, an engaging part 31 of a connecting member 30 can be moved along the linear part 11 in the guide rail 8, and the connecting member 30 keeps the distance between the seat part 5 and the linear part 11 in the guide rail 8 constant, and as a result, the posture of the seat part 5 is kept constant.